The present invention relates to a new process for the preparation of block copolymers in the presence of agents for coupling polymeric blocks, and more particularly involves a process for the preparation of radial block copolymers formed from polyvinylaromatic conjugated polydiene blocks which have been formed using agents comprising epoxy groups.
The present invention also relates to radial block copolymers prepared with new coupling agents and having improved physical properties with respect to those of block copolymers prepared with conventional coupling agents.
The coupling of polymer chains terminated by a lithium atom (also called living based polymers) is well-known in the field, as are the coupling agents used for such purposes. In general, a polymer chain terminated by a lithium atom is reacted with a compound having two or more functional groups capable of reacting with the carbon-lithium bond of the polymer chain.
Depending on whether the intention is to form radial polymers or multi-branched polymers, coupling agents having more than two reactive sites or reaction groups may be used.
European Patent No. 2012 discloses the use of systems of polyfunctional coupling agents in which the addition of a polyvinyl monomer which acts as a non-deactivating coupling agent is used in the first instance, and a di- or trifunctional coupling agent, which may or may not be deactivating, is used in the second instance. This type of system of coupling agents leads to numerous branchings which are difficult to control.
U.S. Pat. No. 4,304,886 discloses the use of mixtures of coupling agents, which may be any coupling agents, in order to obtain a polymer having a desired total functionality greater than 2 without having to blend several polymers. However, the patent teaches that such systems using mixed coupling agents leads to worse mechanical properties.
U.S. Pat. No. 3,880,954 describes the use of alkylpolyalkoxysilane as the sole coupling agent, having at least two and preferably three alkoxy groups. However, this type, of agent alone does not produce the properties desired to be obtained, and moreover it leads to the formation of troublesome by-products, such as the corresponding alcohols.
It is well-known that the residue of the coupling agents remains in the copolymer formed and is thus capable of causing toxic residues or other unwanted products in the polymers, which products may be troublesome in certain uses and, in particular, in the foodstuff packaging sector. This phenomenon is particularly important when one of the coupling agents is silicon tetrachloride (SiCl.sub.4). Indeed, when SiCl.sub.4 is used as coupling agent, lithium chloride (LiCl) is formed as by-product. The presence of LiCl is not only detrimental to the optical properties of copolymers (causing unwanted opacity), but also induces thermal aging of the copolymers.
There is thus a need for the development of a process for the preparation of block copolymers which utilizes coupling agents which do not form toxic or unwanted products, but which are able to achieve the desired physical properties, and especially transparency. Transparency, or improved optical properties are an issue when the copolymers contain less than sixty percent (60%) by weight of the of the mono vinyl aromatic component. The conjugated polydiene block component adversely affects the optical properties of the final copolymer when present in amounts greater than thirty percent (30%) by weight.